The growing demands on cellular communications networks to support data applications at higher throughputs and spectral efficiencies has driven the need to develop Orthogonal Frequency Division Multiplexing (OFDM) based 4th generation (4G) networks including 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). A key objective with respect to deployment of OFDM 4G cellular communications networks is to utilize a frequency re-use of 1 (denoted by N=1), or as close to N=1 frequency re-use as is practical. A frequency re-use of N=1 implies that base stations simultaneously transmit on all available time-frequency Resource Blocks (RBs).
The need for higher throughput in 4G networks, especially near the cell edge, combined with the constraint on the uplink link budget will necessitate the need for smaller cell sizes than is typically deployed for present 2nd generation (2G) and 3rd generation (3G) cellular communications networks. The addition of smaller cells can be deployed in a traditional homogenous cell splitting approach or in a more ad hoc heterogeneous approach in which small cells (e.g., pico cells) or relay nodes are overlaid on a macro cell grid in an existing cellular communications network. For both a homogeneous and a heterogeneous approach, the resulting interference limited system for N=1 deployment will not achieve the full potential capacity that can be supported without the implementation of one or more viable interference mitigation and/or cancellation techniques.
Interference mitigation and cancellation techniques have been investigated and deployed with varying degrees of success in terrestrial cellular communications networks for over 20 years. Traditional approaches to interference mitigation between transmitted signals have focused on either: (i) ensuring orthogonality between transmitted signals in time, frequency, as well as space or (ii) actively removing and cancelling interfering signals from the desired signal if orthogonality between the desired signal and potential interferers cannot be achieved. In early 2G cellular communications networks such orthogonality was achieved primarily through static pre-planned allocations of radio resources. 3G cellular communications networks introduced interference cancellation techniques based mostly on a combination of blind information gathering at a base station (e.g., spectrum usage monitoring) and coarse exchange of interference indicators (e.g., the Rise over Thermal (RoT) indicator employed in the 3GPP2 1×EV-DO standard). Typically, interfering signals have been estimated using blind detection, and then the estimates of the interfering signals are subtracted from the desired signals.
More recently, Coordinated Multipoint (CoMP) transmission and reception approaches have been investigated to improve both cell edge and aggregate user throughputs on both Uplink (UL) and Downlink (DL) transmission in cellular communications networks including LTE release 8, 10, and 11. CoMP can take a number of forms including coordinated scheduling, coordinated beamforming, and joint processing. In particular, whereas joint processing can enhance cell edge coverage and capacity problems, it can result in high inter-base station transmission requirements (i.e., high X2 transmission requirements) for inter-base station CoMP solutions for both homogeneous and heterogeneous networks, including remote-radio head deployments. Thus, one of the fundamental problems of CoMP, which is particularly an issue for UL CoMP but is also an issue for DL CoMP, is the cost and complexity of the needed transport network required to deliver signals between base stations of CoMP coordinated cells and the base station of the CoMP coordinating cell.
One approach for addressing this issue involves transporting digitized Radio Frequency (RF) spectrum signals from Remote Radio Head (RRH) units of neighboring CoMP coordinated base stations to the CoMP coordinating base station. For LTE, this potentially incurs a needed bandwidth of up to 2.5 Gigabits per second (Gbps) per 2 Branch diversity 20 Megahertz (MHz) carrier. After provisions are made to allow multiple cells to share with neighboring cells, the amount of required intercellular bandwidth can potentially exceed the non-CoMP case by a factor of 20. Other choices exist for UL CoMP payload types—see for example U.S. Patent Application Publication No. 2012/0184218. While these payload types are amenable to packet transport and are less demanding for bandwidth, they too may drive high peak bandwidths due to short required latencies.
In the case where the CoMP payload is not a streaming type (i.e., the CoMP payload is not a signal such as RF over Common Public Radio Interface (CPRI) but rather a payload that is arrived at by computations at the cooperating base station), the cooperating base station is under time deadlines to execute the necessary computations and send the result of the computations to the serving base station such that the result arrives within a timeframe in which the serving cell needs the payload. In LTE, the Hybrid Automatic Repeat Request (HARQ) timing for Frequency Division Duplexing (FDD) operation is typically set to 4 milliseconds (ms) such that the HARQ process can assist in exploiting the short term behavior of the mobile channel. This is perceived as a valuable attribute of the air interface. Note that there are even more stringent real time processing requirements for streaming payloads.
Usual solutions are defined that deliver the CoMP payload with a latency that allows the payload to be useful at the serving cell within the HARQ deadline. This drives less than 500 microsecond (μs) latencies and, as a result, high peak data rate requirements. In addition, it should be recognized that, in LTE, the on-air timing of cells are the same such that hand-off and other functions of the air interface are optimized. This compounds the high peak data rates for inter-base station communications.
In addition to the issues discussed above, it has become evident that processing requirements (i.e., Central Processing Unit (CPU) or Digital Signal Processing (DSP) processing requirements) for harvesting a CoMP payload are an issue. Thus, there is also a need for reducing processing requirements for harvesting a CoMP payload.
U.S. Pat. No. 8,305,987, PCT Patent Application Publication No. WO 2012/114151, PCT Patent Application Publication No. WO 2011/020062, and U.S. Patent Application Publication No. 2012/0201191 propose the use of CoMP with mobile relay nodes with regard to a number of aspects including the use of Channel State Information Reference Signal (CSI-RS), carrier aggregation, control, and backhaul signaling. However, these disclosures fail to address interference to cell edge user equipments (and thus decreased throughput to cell edge user equipments), peak data rates, latency, and high processing requirements during CoMP operation.
Thus, there is a need for systems and methods that mitigate peak data rates for inter-base station communications (i.e., X2 communications in LTE) between CoMP coordinated base stations required for CoMP operation. There is also a need for systems and methods for reducing latency of inter-base station communications between CoMP coordinated base stations as well as increasing throughput of cell edge and network aggregate throughput.